whirlpool

When two water currents meet, they sometimes form a swirling, circular whirlpool. There are some small but very strong whirlpools at the bottom of Niagara Falls.

The way water moves in a circle as it drains out of your bathtub is a smaller version of what a whirlpool looks like in a large body of water. Whirlpools can be dangerous for swimmers and small boats, although larger ships aren't at any real risk of being sucked into a whirlpool, even a particularly large and powerful one — which can also be called a maelstrom.